gui: load heatmap before onShow

[LucasYuki]

Summary

Closes #4181

The warning GUI-0066 was caused by calling onShow (which checks if the heatmap is loaded) before loading the heatmap.

Type of Change

• Bug fix

Impact

[How does this change the tool's behavior?]
Now the warning isn't raised.

Verification

• [ X ] I have verified that the local build succeeds (./etc/Build.sh).
• [ X ] I have run the relevant tests and they pass.
• [ X ] My code follows the repository's formatting guidelines.

• I have included tests to prevent regressions.
• [ X ] I have signed my commits (DCO).

Related Issues

[Link issues here]
#4181

[gemini-code-assist]

Code Review

This pull request refactors the rendering logic in heatMap.cpp by extracting the call to getVisibleMap() into a local variable before the loop. However, this changes the execution order, calling getVisibleMap() before datasource_.onShow() on the first paint, which can lead to rendering with uninitialized or stale state. It is recommended to preserve the lifecycle order by explicitly calling datasource_.ensureMap() within the initialization block instead.